Decoupling Control Scheme for VSG-WPPs to Participate in Grid Frequency Response

In several countries, the wind power penetration increased tremendously in recent years. To ensure the proper functioning of the power system, some grid operators already require the capability to provide inertial response or primary control with wind turbines. This paper discusses the emulated inertial response with wind turbines by means of the synthetic inertia and the droop control strategy. The behavior of the synthetic inertia strategy is determined by the inertial and the droop constant, whereas droop control only has a droop constant. When these strategies are used, it is important to tune the control parameters depending on the power system to which the wind turbines are connected. Simulations show that it is possible to enhance but also to deteriorate the frequency response of the system, dependent on these parameters. For different power system compositions, the optimal inertial constant is always close to zero. This way, the synthetic inertia strategy reduces to a fast droop control strategy. This is an important outcome, as it means that no differentiation of the frequency is needed to obtain an optimal inertial response from the wind turbines, which is beneficial in terms of robustness. Consequently, droop control is a viable alternative for the synthetic inertia.

Due to a large number of grid-connected wind turbines don't have the ability to respond to system frequency changes, problems on frequency stability of power system have become increasingly prominent. At present, the primary frequency regulation of wind turbines is mostly based on the method of additional frequency regulation control to change the reference value of electromagnetic power, so as to realize that wind turbines can participate in the system frequency regulation. However, the method of additional frequency regulation control does not t consider the conflict of power increment between the power tracking controller and the frequency regulation controller, which lead to the power increment required for the primary frequency regulation can't reach the target value. Based on this issue, the paper combines the original power tracking technology of wind turbines with the existing frequency regulation technology, and change the signal of frequency deviation by the power tracking curve of the wind turbine, which realize the inertial response and primary frequency regulation of the unit through the fast active power control of the wind turbine. That the purpose of frequency regulation is realized and the power tracking curve is changed, by converting the active output required for the frequency regulation control into the increment load reduction. The simulation results show that, based on the control method in the paper, the double-fed wind turbine can fully achieve the target increment needed for primary frequency regulation, which makes the wind turbines more reliable and effective to participate in system of frequency regulation.

A novel approach for robust control design of hidden synthetic inertia for variable speed wind turbines

A novel grid-forming technology for transient stability enhancement of power system with high penetration of renewable energy

Participation of the wind turbine generators (WTGs) in the frequency regulation service is an appealing issue in order to consider the safe increasing of the wind power generation. The droop and virtual inertia control are the most popular approaches that facilitate the WTGs to provide frequency regulation. However, the intermittent nature of the wind complicates the implementation of these methods and has impacts on the wind turbine stability and may cause violation of the allowed power reserve and minimum turbine rotor speed. Therefore, in this paper, a control approach based on the dynamic de-loading technique is proposed, where the wind turbine operating curve is dynamically adjusted in the response of the frequency deviation throughout controlling the turbine tip-speed ratio which helps the turbine provide steady-state power sharing within the reserved power as well as the transient response within its stability criteria. In addition, the inertial response based on a dynamic gain is suggested. The inertial weighting gain has been formulated where it is continuously regulated in the response of rotor speed and reflects the amount of available kinetic energy in the rotating mass. The effectiveness of the proposed control approaches is verified throughout the comparisons of the results with the fixed inertial gain control and the droop control. The simulation results confirm that the combined control of the proposed tip-speed ratio and dynamic inertia control improve the overall system dynamic behavior in terms of frequency response and turbine stability.

This paper integrates the wind turbines (WTs) into the interconnected power system and designs a distributed model predictive control (DMPC) controller for the primary frequency regulation problem. The WTs have been treated as a part of the power system to take part in primary frequency regulation with load disturbances. A frequency response model of multi-area power system including WTs is introduced, in which the physical constraints of the governors and turbines are considered. The distributed model predictive controller is designed by posing the primary frequency regulation problem as disturbance rejection problem of large system with state and input constraints. Analysis and simulation results for a two-area interconnected power system with WTs show possible improvements on closed-loop performance, while respecting physical hard constraints. Comparisons have been made made between the DMPC controller with and without WTs. The results confirm the superiority of the proposed DMPC scheme with the participation of the wind turbines in the primary frequency regulation.

Measurement-based investigation of inter- and intra-area effects of wind power plant integration

As wind power penetration into power systems increases, it poses a growing challenge to power system operation and stability. This is partly because wind power is highly time variable and cannot inherently contribute to power system frequency regulation. The provision of frequency support from wind turbines under changing wind conditions is examined in this paper. A simplified variable speed wind turbine model is developed. A modified turbine controller to deliver inertial and droop response is proposed. The interaction between inertial and droop response from wind turbines is examined under varying wind.

Estimating the system efficiency of the multifunctional hydrogen complex at nuclear power plants

This paper investigates the possibilities for frequency support by synthetic inertia from variable speed wind turbines. A model for representing synthetic inertia in PSS/E (Power System Simulator for Engineering) has been developed. It allows the user to relatively simple implement different control strategies to find the most suitable strategy for a particular power system. Furthermore, it can be used regardless of the existence of wind turbine models in the power system model and is connected separately to any arbitrary bus in the power system. The results from the simulations show that the frequency nadir is reduced when implementing frequency support by synthetic inertia from wind turbines. As the wind power integration increases the frequency nadir is further improved when wind turbines can provide synthetic inertia. Regarding recovery, the results indicates that in order to minimise the risk of an under-frequency event the energy should be taken during a frequency overshoot and/or distributed in time, i.e. not all wind power plants should recover at the same time. However, these studies of synthetic inertia provided by wind turbines are still in the early stage and further work will be performed.

As the wind power penetration increases, the frequency stability of the power system is deteriorating. Thus, it is expected that the wind power plants (WPPs) will be required to produce the inertial response during the short-term large-scale frequency disturbances. To study the effects of the inertial response of the WPP, the dynamic equivalent model (DEM) of the WPP should be able to emulate the inertial response of the actual WPP. Otherwise, the simulated system frequency would be different from the actual one. Therefore, the inertial response identification algorithm (IRIA) is proposed in this article. The IRIA is able to identify the gain of the inertial controller and the inertia constants of the WPP according to the actual system frequency and the active power output of the actual WPP. In this article, the actual WPP is represented by the detailed model. In order to prove the effectiveness of the IRIA, three cases with different gains of the inertial controller are studied. The simulation results demonstrate that the proposed IRIA allows the DEM of the WPP to generate similar response of the active power and lead to close response of the system frequency as the actual WPP does in three different scenarios.

The different variants of droop controls for the multiple Virtual Synchronous Generator (VSG) sources to form an island microgrid is discussed in this paper. The droop control ω-P is basically a decentralized control that defines the frequency variation of a microgrid on the increase in load. The control equation for the equivalent ω-P slope for two VSGs is derived to see the effect of a load on the overall system's frequency. In the parallel VSG system, the power distribution among the multiple VSGs is dependent on the droop control. The correlation of multiple VSGs is presented in this paper that can bring the firm coordination between the different sources in stabilizing the system; by finding the dependency of each VSG on a microgrid. Furthermore, the master-slave control is introduced to acquire the maximum power of one VSG (e.g. solar) by following the instant frequency of a system as a reference, instead of constant reference frequency. Following controls are verified by performing the experiments on an island microgrid system and the results are shown through graphs.

In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronous-generator-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides inertia support for the system.

Virtual synchronous generator (VSG) is a promising control strategy of an inverter-interfaced distributed generator, owing to its ability to provide inertia support and primary frequency regulation after a grid disturbance. These new features, however, require extra transient energy, which is generally released/absorbed by energy storage (ES). To optimize the design of the ES of a VSG, it is necessary to establish a modeling and calculation method of the transient energy demand (TED) of a VSG and the corresponding maximum grid frequency deviation (MGFD) under a given grid disturbance condition. This method should take the dynamic models of both the grid and the VSG into consideration, including nonlinearity such as droop deadband. Hence, a systematical modeling method and calculation algorithm for the TED and MGFD is presented in this paper. The numerical results obtained by the proposed algorithm are well verified by simulation results from PSCAD/EMTDC. Based on this algorithm, we investigate the influence of VSG parameters on TED and MGFD, and will propose a VSG parameter optimization method to minimize the TED with a given MGFD constraint in our future work.

In contrast to conventional power plants, which are based on large synchronous generators with large inertia capabilities to dampen sudden disturbances, renewable energy sources, such as solar and wind, connected to the grid through power electronics converters, display low system inertia and overload limiting capabilities. Additionally, because they lack primary frequency regulation capabilities, they are unable to actively respond to the frequency response of the system. This research investigates the contribution of droop control strategies to grid resilience by focusing on their ability to maintain frequency stability during grid disturbances. The study employs a simulation-based approach using MATLAB/Simulink to model the Djoum power plant in Cameroun and implement droop control algorithm. The methodology involves designing and analyzing the system's response under sudden load changes using droop and supervisory control strategies. Parameters such as droop coefficients and control bandwidths were systematically varied to analyze their impact on frequency regulation and grid resilience. Results show that droop control maintains system operation by adjusting frequency and voltage under disturbance, while supervisory control acts as a secondary layer to fully restore parameters to their reference values thereby ensuring reliable operation during grid disturbances.